Following the success of the reggeization of various different elementary particles it was hoped that a particle could be identified with the quantum numbers of the Pomeron which would reggeize to give the Pomeron trajectory.

Unfortunately this turned out not to be possible. In particular, in QCD all the elementary particles carry colour so there is no basic QCD constituent with the quantum numbers of the Pomeron. In QCD the lowest order Feynman diagram that can simulate the exchange of a Pomeron is a two-gluon exchange diagram. This led Low (1975) to use two-gluon exchange as a model for the bare Pomeron. He made numerical estimates of the amplitude for the exchange of two gluons between two hadrons using the then fashionable bag model of hadrons. This was then developed by Nussinov (1975, 1976), who considered contributions from more than two exchanged gluons as well as uncrossed ladder corrections to the two-gluon exchange amplitude.

We have already implicitly used the Low–Nussinov model in Chapter 2 to construct the Pomeron in the scalar theory model considered in that chapter. Combining this with our experience in deriving the reggeized gluon we can see what the picture of the Pomeron is in leading logarithm perturbative QCD.

The imaginary part of the amplitude for Pomeron exchange is given in terms of the multi-Regge exchange amplitude for two incoming particles (quarks for simplicity) to scatter into two quarks plus n gluons.